1. Field of the Invention
The invention is related to a method and materials for growing N-face Gallium Nitride (GaN) or M-plane Gallium Nitride using ammonothermal growth techniques.
2. Description of the Related Art
The usefulness of gallium nitride (GaN) and its ternary and quaternary alloys incorporating aluminum and indium (AlGaN, InGaN, AlINGaN) has been well established for fabrication of visible and ultraviolet opto-electronic devices and high-power electronic devices. These devices are typically grown epitaxially on heterogeneous substrates, such as sapphire and silicon carbide, by Vapor Phase Epitaxy (VPE) techniques such as Metal-Organic Chemical Vapor Deposition (MOCVD) and Molecular Beam Epitaxy (MBE). The growth of device layers is usually initiated by growing a buffer layer on the substrate in the MOCVD or MBE reactor. The buffer layer provides a smooth surface of GaN or AlN suitable for successive growth of device layers. However, the buffer layer is usually a Ga-polar (Ga-face) surface, because growth along the N-polar (N-face) direction results in a rough surface in the VPE growth phase.
Commercially available GaN-based devices are all grown on Ga-polar surface (Gallium-face of the C-plane, also known as the (0001) plane). Recently, however, several studies have pointed out many benefits of N-polar (the Nitrogen-face of the C-plane, also known as the (000-1) plane) devices. Also, it has been pointed out that devices grown on M-plane, also known as the {10-10} plane have further advantages over Ga-polar or N-polar devices.
One major benefit of N-polar (N-face) growth is for p-type doping. In Ga-polar (Ga-face) growth of Mg-doped GaN, the film polarity locally starts to invert to the N-polar (N-face) direction. This phenomenon is known as inversion domains when the concentration of Mg exceeds a certain limit. The inversion domains deteriorate the surface smoothness; therefore, a Ga-polar (Ga-face) film is limited in its hole concentrations. Since high Mg doping favors N-polar (N-face) growth, using a N-polar (N-face) substrate is expected to attain higher Mg concentrations, and thus higher hole concentrations. Opto-electronic devices with high p-type conduction will improve their efficiency by decreasing series resistance of the devices.
A second major benefit is the inverted polarization charge. Although GaN-based High Electron Mobility Transistors (HEMTs) are currently available, their usage is very limited due to many unsolved problems. GaN-based HEMTs currently available have high gate leakage and are typically depletion mode devices. Transistors grown on N-face GaN would realize low gate leakage devices, devices that operate in enhancement mode (normally off mode), are crucial for power switching devices, low dispersion devices, and improved carrier confinement.
One of the major benefits of M-plane optical devices is the higher emission efficiency due to absence of polarization field. Another major benefit of M-plane optical devices is that the optically active layer can contain more In, allowing longer wavelength emission. This enables to realize green, yellow, even red color LEDs.
Despite these benefits, current technology is limited to Ga-polar (Ga-face) devices because of the poor surface smoothness of N-polar (N-face) surface or M-plane surface. Therefore new technology to attain a smooth surface of N-polar (N-face) or M-plane GaN is needed to realize the next generation of high-performance devices such as ultra-high bright LEDs, low threshold current Laser Diodes (LDs), high-power high-speed signal transistors and high-power switching transistors.